The purpose of this study is to establish the mechanism(s) of the progression of syringomyelia associated with abnormalities at the craniocervical junction. Measurement of intraventricular pressure, intrathecal pressure, and intrasynrinx press is providing data which elucidate the hydrodynamic mechanism(s) of progression of syringomyelia. Radiographic testing, including magnetic resonance imaging (MRI) flow studies and ultrasonography, is demonstrating how pathologic anatomy alters normal cerebrospinal fluid (CSF) flow. The effect of posterior fossa craniotomy, upper cervical laminectomy, and duraplasty on CSF flow and pressure, syrinx size, and neurological function is being evaluated. Twenty patients have been treated. Only one patient had communication between the 4th ventricle and the synrix. Despite obstruction of CSF pathways at the foramen magnum, phase and cine-MRI demonstrated pulsatile syrinx and cervical subarachnoid CSF flow. Ultrasonographic measurements demonstrated tonsillar descent, cord and syrinx constriction, and caudal syrinx fluid flow during systole. CSF pressure measurements showed that intracranial pulse pressure was transmitted to the cervical subarachnoid space and syrinx. These measurements in patients with syringomyelia were compared with 20 normal volunteers. In the syringomyelia patients, cervical subarachnoid pressure and pulse pressure were elevated and spinal compliance was decreased. CSF velocity was significantly less in the syrinx group than in the normal group, reflecting the anatomical obstruction at the foramen magnum in the synrix group. These results support the theory that patients with syringomyelia have impaired CSF circulation which results in increased pulsatile CSF pressures during the cardiac cycle. Because intracranial pressure is transmitted despite obstruction of the subarachnoid space at the foramen magnum, we conclude that the cerebellar tonsils and the brainstem act on a partially enclosed spinal subarachnoid space to generate cervical subarachnoid VSF pressure waves. These wages compress the spinal cord from without, not from within, as had previously been considered to occur, to propel the syrinx fluid downward with each heartbeat. Syrinx progression occurs as a consequence. Craniocervical decompression and duraplasty improved CSF flow at the foramen magnum in all patients. The syrinx disappeared or decreased in size in all patients after surgery that did not require invasion of the CNS.